Rotor blades of wind turbine are exposed to large dynamic mechanical loads in particular when the wind turbine is operated with a strong wind comprising a high speed. Therefore, the rotor blades of wind turbines and the corresponding supporting structures have been dimensioned such as to be able to withstand large dynamic loads that could occur even if the wind, which is driving the wind turbine, is strong. However, in case of extreme wind conditions the wind load on structural components of the wind turbine must be reduced in order to avoid any damage of the wind turbine.
Presently, there a known numerous methods of handling wind turbines at high wind situations. Thereby, the term “handling wind turbines” means operating the wind turbine in such a manner that the various mechanical loads acting on structural components such as for instance rotor blades of the wind turbine are kept within safe limits.
An existing very conservative measure by which the wind load on a wind turbine is reduced in high wind conditions is simply shutting down the wind turbine when the wind speed exceeds one or more threshold values. Thus a wind turbine may be shut down when the wind speed exceeds the value of about 25 m/s for about 10 minutes, when the wind speed exceeds the value of about 28 m/s for about 30 seconds or when the wind speed exceeds the value of about 32 m/s for about 1 second.
The problem with this conservative measure is when many wind turbines are hit by high wind at the same time, the power generation of all involved wind turbines goes from a full production to a zero production within a short time. Such a rapid change represents a serious challenge for the stability of the electricity grid, because the rest of the electrical suppliers of the grid need to ramp up very quickly in order to avoid an under frequency situation due to an electrical overloading.
EP 0 847 496 B discloses another measure for limiting mechanical loads acting on a wind turbine. Thereby, when a wind velocity is reached which is in danger of mechanically overloading the wind turbine, the operating speed of the rotor is continuously reduced in dependency of the rise in the wind velocity.
US 2007/216166A discloses a method for reducing the power output or the rotor speed of a wind turbine. Thereby, the amount of reduction is not based simply on a measured wind speed but on the actual pitch angle of one or more rotor blades. The blade pitch angle is an input value, which on one hand can be easily detected physically and by appropriate control technology and on the other hand is a good indicator of mechanical stresses on the wind turbine.
WO 2001/33075 A1 discloses a method of controlling the operation of a wind turbine, wherein mechanical loads acting on the blades of the rotor of the wind turbine are measured. By controlling the pitch angle of the blades the measured mechanical loads can be maintained below certain limits during the wind turbine operation.
EP 2 096 301 A2 discloses method for operating a wind turbine plant comprising a plurality of wind turbines each having a turbine rotor including at least one blade mounted to a rotatable turbine shaft and an induction generator having a stator and a rotor coupled to the turbine shaft for rotation therewith. A wind speed is measured and a power derating factor is determined in response to the measured wind speed. The determined derating factor is communicated to the plurality of wind turbines. The amount of power generated by each of the plurality of wind turbines is then adjusted in response to the power derating factor.
There may be a need for providing an efficient and flexible control procedure for the operation of a wind turbine, which, at least in some strong wind conditions, allows for keeping the wind turbine in operation.